<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8" />
  </head>
  <body>
    <script type="text/javascript">
      const _instanceof = (target, Fn) => {
        // 补全代码
        if (target.__proto__ === Fn.prototype) {
          return true
        }
        //if(null)----false
        if (Fn.prototype) {
          _instanceof(target, Fn.prototype)
        } else {
          return false
        }
      }
    </script>
  </body>
</html>
